It varies by state a location. In rural Michigan and Arizona we charged $1200-1600 per month for rent spelled out in a lease. The cell company would also maintain a bond to pay for the cost of removal of their equipment if they let it go.

Likely they are attaching antennas, rather than installing a tower. Depending on height and demand, it might be possible to place as many as four carriers' equipment on the one tower. They like 10' vertical displacement so as to avoid any interference issues. Some WT can be retrofitted with a "high hat" to provide antenna mounting space.

I worked an install on a brand-new water tank. The muni had built in conduit for the future cable runs, and they'd already installed a paint rail for ease of installation.

This is going off memory since it's been six months since I glanced at the charts hanging on my former office wall.

Lease rates (A-company, 2006, what we tried to negotiate it to)
Raw land build $700/month
Rooftop $1200 and up
Water tank $1500
Collocation on someone else's tower $1800

Suggest you work in some favorable lease terms. Most carriers won't provide equipment (cell phones) for free, but they will pay more rent money and/or a discount on same. You'll need to provide them with substantial advance notice for painting, and allow them to put up a temporary tower (called a COW) while your tower is unavailable.

Get them to paint the cables to match the tower. There's one near me with a massive black snake of coax lines running up the side of an otherwise white WT.